In the production of oil, gas and other hydrocarbons, a tubing string is often positioned within the well casing. The hydrocarbons enter the tubing through perforations located at the lower end of a tubing string. In some wells, the hydrocarbons are pumped to the surface with a sucker rod pump located on the surface or with a downhole submersible pump. At the well surface, production equipment directs the hydrocarbon fluids to holding tanks or to a pipeline. The well production equipment typically comprises tubing, valves, piping, and other components.
The hydrocarbon fluids contain numerous compounds which adversely affect the well production equipment. For example, paraffins and water/oil emulsions can coat the well production equipment and eventually plug perforations in the tubing. In addition, chemical reactions between the hydrocarbon fluids and metallic equipment can cause scale to be formed on the well production equipment, and corrosive compounds in the hydrocarbon fluids can physically corrode the well production equipment.
Various techniques can treat these well conditions to extend the useful life of the well production equipment. In wells susceptible to paraffin build-up, "treater trucks" are regularly dispatched to pump hot oil into the well. The hot oil enters the casing, melts the paraffin deposits in the well production equipment, and returns to the surface through the tubing. For wells susceptible to corrosion and scale problems, high pressure injection trucks pump batches of chemicals into the well to chemically remove the scale, and to inhibit the causes of corrosion. All of these practices require regular maintenance services which are costly and which do not continuously treat the well. Batch treatment of wells is less efficient than continuous treatment because more chemicals are typically injected in batch treatment operations.
To avoid inefficiences associated with treater truck maintenance of hydrocarbon producing wells, well operators use mechanical pumps to inject chemicals into a well. Typically, the mechanical pumps are supplied from a storage tank which holds the chemicals. The mechanical pumps and storage tanks are located adjacent the well for several reasons, such as for reducing the length on the power cable connected to the pump. The tanks are located above the pump and the chemical is gravity fed to the intake port of the pump. These tanks include a vent at the upper end of the tank to prevent a vacuum from developing in the tank as the pump draws chemical from the tank. In addition, the vent releases excess pressure within the tank caused by thermal expansion of the chemical. Such thermal expansion can cause the chemical vapors to be released into the environment through the vent. In addition, thermal expansion can cause the chemical to be ejected through the vent or through the sight glass used to indicate the chemical level in the tank. In either event, chemical vapors or the chemical fluids are released in an uncontrolled manner and can pose a hazard to personnel and to the environment.
The mechanical pumps used in chemical injection systems are powered by electricity or gas and include numerous moving components. It is customary to inspect these pumps on a regular basis, sometimes daily, to verify the operability of the pumps. Because the chemical is gravity fed to the intake of the chemical pump, sediment in the tank or the chemical settles toward the pump intake and can interfere with the operation of the pump. In addition, the presence of an air bubble in the intake line can impede the operation of the pump because of a vapor lock. In such event, maintenance personnel routinely open a bleeder valve on the pump and release chemical from the pump until the air bubble has been cleared. This practice is undesirable because it releases chemical into the environment.
Presently available systems contain moving components which are subject to failure and require regular maintenance. Such systems are also undesirable because they vent chemicals into the environment. Accordingly, a need exists for a system which injects chemicals into a hydrocarbon producing well without moving components and without releasing the chemicals into the environment.